What are you tightening?
i.e. should be loosen stem bolts, then tighten top cap bolt to draw it up. Then tighten stem bolts. Make sure it’s all drawn together properly.
If still an issue then the immediate suspect would be the headset bearings, or at least one of them. Do you get grinding sounds or feel when rotating the bars?
Fork – grab the lower and upper and check for play (don’t grab the arch). If there’s play then likely fork bushing worn. Unlikely to be seal.
Another is brake calliper mount or brake disc loose.
Headset though, just apply front brake (not rear) and gently rock bike back and forwards and look closely at the headset or use fingers to feel if it’s moving. If it does, then yes it’s the headset.